Interplay between host innate immune responses and viral antagonism determines the effectiveness of viral replication and pathogenicity. Influenza A virus (IAV) confronts host antiviral responses including type I interferon (IFN) production using multiple viral proteins. PACT is a double-stranded RNA-binding protein implicated in host-IAV interaction. On one hand, its activation of RIG-I-dependent type I IFN production is inhibited by viral non-structural protein NS1. On the other hand, PACT is one of cellular proteins identified to interact with IAV RNA polymerase subunit PA. Exactly how PACT exerts its antiviral activity during IAV infection remains to be elucidated. In this study, we demonstrated mutual antagonism of PACT and IAV polymerase. PACT activated RIG-I in the induction of IFN-β by IAV ribonucleoprotein complex. PACT-dependent activation of IFN-β production was suppressed by IAV polymerase subunits PA, PB1 and PB2. PACT associated with PA, PB1 and PB2. Compromising PACT in IAV-infected A549 cells resulted in the augmentation of viral RNA transcription and replication as well as IFN-β production. Furthermore, viral RNA replication was boosted by knockdown of PACT in both A549 cells and IFN-deficient Vero cells. Thus, the antiviral activity of PACT against IAV is mediated primarily through its interaction with and inhibition of viral polymerase. Taken together, our findings reveal a new facet of host-IAV interaction in which PACT and viral RNA polymerase antagonize each other to regulate viral replication and host antiviral response.
Supported by RGC (HKU1/CRF/11G, N-HKU712/12, T11-707/15-R, C7011-15R and ECS-27121515), HMRF (12111312, 14130862, HKM-15-M01 and 15140662) and S. K. Yee MRF(2011).
Research Assistant Professor
The University of Hong Kong